Control of anterior pituitary hormone secretion involves: 1) positive feedback (e.g., estrogen stimulation of LH and FSH), 2) negative feedback by target gland hormones (e.g., T4-T3 suppression of TSH), 3) direct central nervous system modulation (e.g., diurnal variation of ACTH, TSH) and, 4) a direct feedback of the hormone on its own secretion, so called "short-loop feedback control" (e.g., LH on LH). In order to better define the last mechanism of control, we have taken advantage of the striking species specificity of a rabbit LH radioimmunoassay to directly demonstrate short-loop feedback. In these studies, highly purified human LH was injected into unanesthetized castrate rabbits, bearing chronically implanted venous catheters and rabbit LH was quantified. The rabbit LH assay does not react with human LH, though both hormones have biological activity in the rabbit. Our studies, performed over the past two years, demonstrated the following: 1) Short-loop feedback control of LH is specific, i.e., LH has no effect on FSH secretion, nor on the secretion of any other hormone studied. 2) Short-loop feedback control of LH is exquisitely sensitive, responding to blood concentrations of human LH present in eugonadal men or women. 3) Most recently we have shown the site of action of short-loop feedback control is the pituitary gland itself. The studies proposed herein are a continuation of these studies on LH. Using the model we have now established, we propose to similarly study short-loop control of FSH, TSH, GH, prolactin, ACTH, lipotropin and endorphin, and the interrelations of these hormones on each other. Our model permits repeated study of the same animal over several weeks' time, each animal serving as its own control. For each hormone, the following will be specifically determined: 1. Biological effectiveness of the foreign species of hormone in the rabbit. 2. Presence or absence of short-loop feedback control. 3. Specificity of control, if it exists. 4. Sensitivity--complete dose response data will be constructed. 5. Anatomic site of short-loop feedback.